Constant resistance coupling arrangement



July 5, 1949. H. A. WHEE LER 2,475,344

CONSTANT RESISTANCE COUPLING ARRANGEMENT Filed Jan. 4, 1947 ll 29 28 R FIG. 4

INVENTOR. HAROLD A. WHEELER ATTORNEY Patented July 5, 1949 CONSTANT RESISTANCE COUPLING ARRANGEMENT Harold A. Wheeler, Great Neck, N. Y., assignor to Hueltine Research, Inc., Chicago, 111., a corporation oi. Illinois Application January 4, 1947, Serial No. 720,168

This invention is directed to coupling arrangements which have a substantially constant .resistance between two terminals thereof over a predetermined range of wave lengths and, more particularly, to coupling arrangements which employ wave guides to develop between two terminals of the arrangement a resistance which is independent ofthe operating wave length.

The so-called constant resistance network is generally an arrangement of series and shunt impedance elements, which ordinarily include reactances, that are combined in such a way that the product of the impedance Z1 of one arm thereof and the impedance Z: of the other arm is a constant R wherein R represents a resistance associated with each arm of the network. If this relation is fulfilled, then the input impedance of the network is independent of the operating wave length thereof and is equal to a pure resistance having a value R. For this reason such networks are employed as coupling arrangements to translate energy between two circuits.

Heretofore the use of known types of constant resistance networks for translating wave-signal energy has been limited to long wave-length ap-, plications. For many short wave-length applications, for example in short wave-length mutualinductance type piston attenuators wherein accuracy is a requisite, it is important that the coupling arrangement or pickup loop structure thereof afford a constant resistance over the range of wave lengths for which the attenuator may be employed. This is to assure that the coupling arrangement presents an impedance which matches that of the transmission line associated therewith. Prior coupling arrangements of this type have not possessed this desirable characteristic.

It is an object of the invention, therefore, to provide'a new andimproved coupling arrangement which is adapted for use at short wave lengths.

It isan additional obiect of the invention to.

provide a new and improved coupling arrangement which is adapted for use in connection with short wave-length mutual-inductance piston attenuators.

In accordance with a particular form of the invention, a coupling arrangement having a substantially constant value of resistance between two terminals thereof over a predetermined range of wave lengths comprises two coaxial transmission-line sections having substantially equal eifective lengths which are substantially integral multiples of one-quarter wave length at a pre- 5 Claims. (Cl. 178 -44) determined wave length in the above-mentioned range. Each transmission-line section has a predetermined wave impedance and has a remote end and a junction end, the remote ends being positioned in spaced relationship and the junction ends in adjoining relationship with the inner conductor of one of the sections forming a continuation of the inner conductor of the other section. The coupling arrangement also includes a low-impedance termination at the remote end of one of the sections and forming therewith an inductive coupling element and a high-impedance termination at the remote end of the other of the sections, thereby providing inverse variations of the impedances at the junction ends with variations in wave length. The network further includes a first resistive impedance, having a value which is equal to the geometric mean value of the aforesaid wave'impedances, connected across the junction end of one of the sections, and also includes a second resistive impedance having the aforesaid geometric mean value of wave impedances connected across the junction end of the other of the sections. As a result, the resistive impedances are connected in series relationship with each other by virtue of the common inner conductor for the transmission-line sections and the uncommon terminals of the resistive impedances provide the first-mentioned two terminals.

For a better understanding of the present invention, together with otl" er and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring now to the drawing, Fig. 1 is a circuit diagram of an attenuator which includes a coupling arrangement embodying the present invention in a particular form; Fig. 2 is a circuit diagram of a modified form thereof; Fig. 3 is an axial sectional view of an attenuator of the Fig. 1 type including a coupling arrangement in accordance with a particular form of the invention; Fig. 4 is a similar view of an attenuator of the Fig. 2 type; and Fig. 5 is a sectional view along the line 5-5 of Fig. 4.

Referring now more particularly to Fig. 1 of the drawing, there is represented schematically an attenuator including a coupling arrangement or network Ill in accordance with the invention. The arrangement [0 has a substantially constant value of resistance between two terminals thereof, specifically terminals II, II, over a predetermined range of wave lengths. The arrangement I0 includes two wave guides "and I3 having substantially equal effective lengths which are substantially an integral multiple of one-quarter wave length at a predetermined wave length in the aforesaid range. For simplicity and compactness of construction the wave guides i2 and ii are preferably approximately one-quarter of the minimum operating wave length of the attenuator. The wave guides l2 and I3 may comprise conventional hollow wave guides for the translation of signals having extremely short wave lengths or may comprise parallel-wire transmission lines of the open-wire or coaxial-type construction for the translation of signals having moderately short wave lengths. Parallel-wire transmission lines have been represented to simplify the illustration. The wave guides I2 and I3 are coupled in series relationship between the terminals ii, Ii Each of the wave guides l2 and I3 has a predetermined wave or characteristic impedance which is established in the well-known manner by the geometry of the wave guide. The wave impedances for the wave guides l2 and II are designated R and R", respectively, and these impedances may be equal or different, as determined by the dimensions of the particular wave guides which are employed.

The wave guide i2 has a remote end l5 and a Junction end l6 while the wave guide It has corresponding remote and junction ends I! and i 8, respectively. The remote ends l5 and I! are terminated with opposite reflection coefilcients to provide inverse variations of the impedances designated Z1 and Z2 at the junction ends I6 and I8, respectively, with variationsof the operating wave length of the coupling arrangement i0. Accordingly, the wave guide I! is terminated at its remote end I! with a low-impedance connection or short-circuit while the remote end l5 of the wave guide i2 is open-circuited. The coupling arrangement l0 also includes individual resistors and 2|, each having a, value R which is equal to the geometric mean value of the wave impedances R and R" of the wave guides i2 and i3,'respectively, coupled between individual ones of the junction ends i6 and I8 and coupled in series relationship with each other between the aforesaid terminals. Accordingly, the resistor 20 is connected in shunt relationship with the junction end l6 of the wave guide l2 while the resistor 2i is connected in similar relationship with the junction end l8 of the wave guide Ill.

The attenuator also includes a first circuit element comprising a signal generator 25, the wave length of the output signal of which is variable over a range of operating wave lengths, which is connected to a coupling loop 26 that is inductively coupled to the wave guide I3. The coeflicient of coupling for the members 26 and i3 is preferably much less than unity. The wave guide i 3 and the coupling loop 26 are arranged for relative movement so that the extent of the inductive coupling therebetween may be varied in the well-known manner. The terminals H, H of the coupling arrangement in are adapted to be coupled to a terminating impedance 28 for the network, either directly or through a transmission-line section 29. The terminating impedance 28 is a pure resistance having a value designated R which is equal to that of the resistors 20 and 2| and also to that of the transmission-line section 29.

Considering now the'operation'of the arrangement of Fig. 1, it will be assumed initially that the generator is develop ng a signal of the desired voltage and wave length and that the position of the wave guide I: relative to the coupling loop 25 has been adjusted to afford the desired extent of inductance coupling therebetween. Accordingly, the voltage produced by the generator 25 is reduced a known amount by the signal over the operating range of the network.

It is known in the art that, for the condition of substantially constant resistance R in a twoarm network with variations of the wave length of the applied signal, the arms of the network must be constructed so that their respective impedances Z1 and Z: satisfy the relation Z1Z2=R, where R is a pure resistance which also corresponds to the termination of the network. Pairs of impedance elements in difl'erent arms of the network, for example an element in the series arm and the related element in the shunt arm, which fulfill this relation are termed inverse elements. In the Fig. 1 arrangement, the wave guides i2 and I 3 constitute inverse elements. This pertains since the guides have equal effective lengths which are an integral multiple of onequarter of a predetermined wave length in the operating range of the network and have terminations at the remote ends l5 and I1 thereof which are opposite in character. Thus the wave guides l2 and i3 are effective to provide inverse variations of the impedances Z1 and Z2 at the junction ends i6 and i8 with variations in the wave length of the signal applied to the wave guide l3 by the generator loop combination 25,

26. Since the wave guides i2 and I3 have equal effective lengths, they have the same resonant wave length. At resonance the wave guide I! acts as a series-resonant circuit and presents a low or zero impedance at its junction end I B so 0 that the resistor 20 is effectively short-circuited by the impedance Z1. Because the wave guides l2 and [3 are constructed to exhibit reciprocally related resonant characteristics, the wave guide It simulates a parallel-resonant circuit at resonance and affords a very high impedance Z: at the junction end l8. Consequently, the value of the resistor 2i which is connected in shunt with the junction end I 8 is effectively unaltered by the wave guide i 3 at resonance and a resistance 0 having a value equal to R is connected between the terminals ii, ii. Since the wave guides i2 and i3 have opposite impedance characteristics not only at resonance but also at wave lengths differing from the resonant wave length, a compensating action results for signals which differ from the resonant wave length so that a substantially constant resistance R appears between the terminals H, II of the network regardless of the wave length of the signals translated thereby. Best operation of the network is assured when the approximate minimum operating wave length is four times the effective length of the wave guides i 2 and I3. Because loose coupling is afforded between the network It and the coupling loop 26 in circuit with the generator 25,

the impedance of the generator has a negligible reaction on the impedance of the network and vice versa. Consequently, the coupling arrangement l6 eflectively provides a uniform resistance over its operating range.

Referring now to Fig. 2 of the drawing, there is represented a, coupling arrangement which is generally similar to the arrangement of Fig. 1, corresponding elements of the former being designated by the same reference numerals primed. In the Fig. 2 embodiment each wave guide and its corresponding resistor are connected in series relationship between the terminals II, II. Accordingly, the wave guide and resistor combination I2, 20' is connected in shunt relationship with the other wave-guide resistor combination Operation of the attenuator of Fig. 2 including the coupling arrangement I! is similar to that of the attenuator including the coupling arrangement ill of the Fig. 1 embodiment. Since the wave guides i2 and I3 constitute inverse elements at resonance, the wave guide l3 presents a high impedance or open circuit at its junction end 18' so that the resistor 2| is effectively disconnected from the terminals H, H' while the resistor 20' only is eflectively connected between the aforesaid terminals since a low impedance or short circuit is present at the junction end i6 of the wave guide l2. For ofl-resonance conditions the compensating action provided by the wave guides is similar to that afforded by corresponding wave guides of the Fig. 1 arrangement.

Referringnow to Fig. 3 of the drawing, there is illustrated a longitudinal sectional view of a piston attenuator which includes a coupling arrangement 30 which is connected in accordance with the circuit diagram of Fig. 1. A signal generator 3i is connected to a coupling loop 32 which is disposed in one end of a fixed hollow conductive cylinder 33. The coupling arrangement 30 includes two .wave guides in the form of coaxial transmission-line sections 35 and 36 which have equal effective lengths that are preferably onequarter of a predetermined wave length in the operating range of the arrangement. Transmission-line sections 35 and 36 correspond to the wave guides l2 and I3, respectively, of the Fig. 1 arrangement and, accordingly, have wave impedances R and R", respectively. The outer conductor of transmission-line section 36 is a hollow cylinder 31 which is slidable within the fixed hollow cylinder 33 for a purpose to be explained subsequently. The inner conductor 36 of coaxial transmission-line section 36 is conductively attached at one end to the inner surface of the outer conductor 31 by an end portion 36, thus forming a short-circuited remote end which affords inductive coupling with the coupling loop 32 and, hence, corresponds to the remote end ll of the Fig. 1 embodiment. The other end of the transmission-line section 36 corresponds to the junction end I8 of the Fig. 1 arrangement and is terminated with a pair of resistors 40, 46 which are preferably connected between diametrically opposite points on the inner surface of the outer conductor 31 and a point on the inner conductor 38. This group of resistors has a resultant value which is equal to the geometric mean value of the wave impedances R and R" and, hence, has an effective value R. Thus, the pair of resistors 46, 4!) correspond to the resistor 2| which is connected across the junction end l3 of the wave guide l3 of Fig. 1.

Transmission-line section 35 includes an inner 1 6 conductor 42 which comprises an axially aligned continuation of the inner conductor 33 of the transmission-line section 36. Insulating material maintains the inner conductor 42 in spaced coaxial relationship within the outer conductor 44 of thetransmission-line section 35. The outer conductor 44 also forms the inner conductor of an additional coaxial transmission-line section 46 which has a wave impedance R and corresponds to .the transmission line 26 of Fig. 1. The outer conductor of transmission-line section 46 is an extension of the outer conductor 31 of transmission-line section 36. Transmission-line section 46 is connected to a pair of terminals 41, 41 through a tapered transmission-line section 43 which is effective to maintain the desired wave impedance R.

The junction ends of transmission-line sections 35 and 36 are disposed in adjoining relationship. Accordingly, a pair of resistors 50, 50 are positioned in close proximity to the resistors l6, l6 and are connected between diametrically opposite points on the outer conductor 44 and a point on the inner conductor 42 of transmissionline section 35. The resistors" 50, 50 also have a resultant value which is equal to the geometric mean value of the wave impedances R and R" and, hence, have an effective value It like the corresponding resistor 26 in the Fig. I arrangement. It will be manifest that the remote ends of transmission-line sections 35 and 36 are disposed in spaced relationship and that inverse variations of the wave impedances are provided at the junction ends near the resistors 40, 46 and 50, 50 with variations in the operating wave length, just as in the Fig. 1 arrangement. It will also be apparent that the physical lengths of the transmission-line sections 35 and 36 may vary somewhat due to the diflerent dielectric constants of the insulation between the inner and outer conductors thereof. It will also be manifest that movement of the cylinder .31 with respect to the fixed cylinder 33 is effective to adjust the extent of the coupling between the coupling loop 32 and the end portion 39 of the network 36. A suitable scale (not shown) may be employed to indicate the position of the movable outer conductor 31 and, hence, the position of the network 30, with relation to the coupling loop 32.

Since the Fig. 3 arrangement represents a physical embodiment of the attenuator of Fig. 1 and its similarity is readily apparent, it is deemed unnecessary to repeat the operation thereof.

Considering now the arrangement of Fig. 4, there is represented an axial sectional view of a physical structure corresponding to the schematic arrangement of Fig. 2. A coupling arrangement 66 is adjustable axially within a, hollow cylinder 63 with respect to a fixed coupling loop 62 that is connected to a signal generator 64. The arrangement includes a hollow conductor 6| which serves as the major portion of the outer conductors for two transmission-line sections 65 and 66 which have wave impedances R and R", respectively, and eflective lengths which are preferably one-quarter of a predetermined wave length in the range of wave signals which are adapted to be translated by the arrangement 66. An electrostatic shield 66 (see also Fig. 5) extends across the diameter of the hollow conductor 6| and is conductively secured thereto by any suitable means such as brazing to form two chambers. Shield 66 has approximately the same 15 length as the inner conductors 61 and 69 of the transmission-line sections 66 and 66, respectively.

Inner conductor 69 is supported in the chamber of the transmission-line section 66 by means of a block of insulating material in such a manner that the conductor 69 does not make a conductive connection with the cylinder 6! or the shield 68 throughout its length. A conical conductive member II is connected between one end of the hollow conductor 6| and a coaxial transmission line 12 which includes inner and outer 10 conductors I3 and M, respectively. Conductors i3 and 14 are provided with output terminals 15, 15. Transmission line. 12, which has a wave impedance R, therefore corresponds to the transmission line 29' of Fig. 2. A resistor 16, which has a value R that is equal to the geometric mean value of the wave impedance R. and R";

is connected between one end of the inner conductor 69 of transmission-line section 65 and the inner conductor 13 of transmission line 12. It will be evident that the transmission-line section 65 corresponds to the wave guide i2 illustrated in F g. 2 and that the resistor 16 is the counterpart of the resistor 20' of Fig. 2.

The junction end of transmission-line section 66, which issupported by an insulating disc 11, is connected to the conductor 13 of the transmission line 12 by means of a resistor 18 which also has a value R. The remote end of transmission-line section 66 includes an end portion 80 which connects the inner conductor 61 with the inner surface of the outer conductor 6|, thereby forming a coupling loop which aflords inductive coupling with the fixed coupling loop 62. It will be seen that the transmission-line section 66 and the resistor 18 which is connected to the junction end thereof are the counterparts of the wave guide i3 and the resistor 2|, respectively, of the Fig. 2 arrangement.

Considering briefly the operation of the Fig. 4 attenuator, magnetic flux developed by the coupling loop 62 links the end portion 80 of the transmission-line section 66 and a voltage is induced in the latter so that an output signal is developed at the terminals 15, I5 of the coupling arrangement 60. The principal coupling with the coupling loop 62 by elements of the constant resistance network 60 is afforded by the transmission-line section 66. However, a limited amount of capacitive coupling takes place at the remote end of the open-circuited transmission-line section 65. Since the currentinduced in the latter is of reverse polarity with respect to that derived by the transmission-line section 66, its action isin aiding relationship. The electrostatic shield 68 is effective to isolate the two transmission-line sections 65 and 66 while permitting the magnetic flux lines to penetrate the two chambers in the hollow conductor 6|. As explained in connection with the previously described arrangement l0 of Fig. 2, the arrangement 60 of Fig. 4 aifords a pure resistance which is substantially independent of frequency.

From the foregoing description it will be manifest that a coupling arrangement embodying the present invention affords a substantially pure constant resistance which is effectively independent of the wave length of the signal translated thereby over a larger range of operating wave lengths than has heretofore been possible with such arrangements.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modi- 8 flcations may be made therein without departing from the invention, and it is, therefore,-

aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A coupling arrangement having a substantially constant value of resistance between two terminals thereof over a predetermined range of wave lengths comprising: two coaxial transmission-line sections having substantially equal effective lengths which are substantially integral multiples of one-quarter wave length at a predetermined wave length in said range, each thereof having a predetermined wave impedance and having a remote end and a junction end, said remote ends being positioned in spaced relationship and said junction ends in adjoining relationship, the inner conductor of one of said sections forming a continuation of the inner conductor of the other of said sections; a low-impedance termination at the remote end of one of said sections and forming therewith an inductive coupling element and a high-impedance termination at the remote end of the other of said sections, thereby providing inverse variations of the impedances at said junction ends with variations in wave lengths; a first resistive impedance, having a value equal to the geometric mean value of said wave impedances, connected across the junction end of said one of said sections; and a second resistive impedance, having said geometric mean value of said wave impedances, connected across the junction end of said other of said sections, whereby said resistive impedances are connected in series relationship with each other by virtue of said common inner conductor and the uncommon terminals-of said resistive impedances provide said two terminals.

2. A coupling arrangement having a substantially constant value of resistance between two terminals thereof over a predetermined range of wave lengths comprising: two coaxial transmission-line sections having substantially equal effective lengths which are substantially integral multiples of one-quarter wave length at a predetermined wave length in said range, each thereof having the same values of wave impedance and having a remote end and a junction end, said remote ends being positioned in spaced relationship and said junction ends in adjoining relationship, .the inner conductor of one of said sections forming a continuation of the inner conductor of the other of said sections; a lowimpedance termination at the remote end of one of said sections and forming therewith an inductive coupling element and a high-impedance termination at the remote end of the other of 9 said sections, thereby providing inverse variations of the impedances at said junction ends with variations'in wave length; a first resistive impedance, having a value equal to the geometric mean value of said wave impedances, connected 55 across the junction end of said one of said sections; and a second resistive impedance, having said geometric mean value of said wave impedances, connected across the junction end of said other of said sections, whereby said resistive 7 impedances are connected in series relationship with each other by virtue of said common inner conductor and the uncommon terminals of said resistive impedances provide said two terminals.

I 3. A coupling arrangement having a substan 76 tially constant value of resistance between two terminals thereof over a predetermined range of wave lengths comprising: two coaxial transmission-line sections having substantially equal efiective lengths which are one-quarter wave length at approximately the minimum wave length in said range, each thereof having a predetermined wave impedance and having a remote end and a junction end, said remote ends being positioned in spaced relationship and said junction ends in adjoining relationship, the inner conductor of one of said sections forming a continuation of the inner conductor of the other of said sections; a low-impedance termination at the remote end of one of said sections and forming therewith an inductive coupling element and a high-impedance termination at the remote end of the other of said sections, thereby providing inverse variations of the impedances at said junction ends with variations in wave length; a first resistive impedance, having a value equal to the geometric mean value of said wave impedances, connected across the junction and of said one of said sections; and a second resistive impedance, having said geometric mean value of said wave impedances, connected across the junction end of said other of said sections, whereby said resistive impedances are connected in series relationship with each other by virtue of said common inner conductor and the uncommon terminals of said resistive impedances provide said two terminals.

4. A coupling arrangement having a substantially constant value of resistance between two terminals thereof over a predetermined range of wave lengths comprising: two coaxial transmission-line sections having substantially equal efiective lengths which are substantially integral multiples of one-quarter wave length at a predetermined wave length in said range, each thereof having a predetermined wave impedance and having a remote end and a junction end,

said remote ends being positioned in spaced relationship and said junction ends in adjoining relationship, the inner conductor of one of said sections forming an axially aligned continuation of the inner conductor oi the other of said sections; a low-impedance termination at the remote end of one of said sections and forming therewith an inductive coupling element and a high-impedance termination at the remote end 01' the other of said sections, thereby providing inverse variations of the impedances at said junction ends with variations in wave length; a first resistive impedance, having a value equal to the geometric mean value oi said wave impedances, connected across the junction end of said one of said sections: and a second resistive impedance. having said geometric mean value oi said wave impedances, connected across the junction end of said other of said sections, whereby said resistive impedances are connected in series relationship with each other by virtue of said common inner conductor and the uncommon terminals oi said resistive impedances provide said two terminals.

5. A coupling arrangement having a substantially constant value of resistance between two terminals thereof over a predetermined range of wave lengths comprising: two coaxial transmission-line sections having substantially equal effective lengths which are one-quarter wave length at approximately the minimum wave length in said range, each thereof having a predetermined wave impedance and having a remote end and a junction end, said remote ends being positioned in spaced relationship and said junction ends in adjoining relationship, the inner conductor of one of said sections forming a continuation oi. the inner conductor of the other of said sections and the outer conductor of said one section having an extension enclosing the outer conductor of said other section and forming therewith a third coaxial transmission-line section having a wave impedance equal to the geometric mean value of said first-mentioned wave impedances; a low-impedance termination at the remote end of one of said sections and forming therewith an inductive coupling element and a high-impedance termination at the remote end of the other of said sections, thereby providing inverse variations of the impedances at said junc-- tion ends with variation in wave length; a first resistive impedance, having a value equal to the geometric mean value of said wave impedances, connected across the junction end of said one of said sections; and a second resistive impedance, having said geometric mean value of said wave impedances, connected across the junction end of said other of said sections, whereby said resistive impedances are connected in series relationship with each other by virtue of said common inner conductor and the uncommon terminals of said resistive impedances provide two terminals for connection to said first-mentioned terminals through said third section.

HAROLD A. WHEELER.

REFERENCES CITED UNITED STATES lATENTS Name Date Number West Jan. 20, 1942 

